Cavity expanding and contracting foam injection molding method

Abstract

A foam injection molding method which makes it possible to control physical properties of high-expansion ratio molded parts with a high degree of freedom of interfacial configuration. With a molding material charged in a closed initial cavity (8) formed by three mold elements (1, 2 and 3), an intermediate cavity having a smaller volumetric capacity than that of the initial cavity (8) and an intermediate cavity having a larger volumetric capacity than that of a final cavity (9) are formed by moving at least one mold element (3) relative to the other two mold elements (1 and 2) without moving these mold elements (1 and 2) while keeping the cavity closed, which is formed by the three mold elements (1, 2 and 3).

What is claimed is: 
     
       1. A foam injection molding method comprising the steps of: (a) forming a closed initial cavity by using at least three mold elements of which one of at least two mold elements is movable relative to the other two mold elements,   (b) charging a molding material into the initial cavity formed by said cavity forming step;   (c) forming an intermediate cavity having a volumetric capacity different from that of said initial cavity by moving one of said three mold elements relative to the other two mold elements without moving said other two mold elements while keeping the cavity closed, which is formed by said three mold elements; and   (d) forming a final cavity having a larger volumetric capacity than that of said initial cavity by moving one of said three mold elements relative to the other two mold elements without moving said two mold elements while keeping the cavity closed, which is formed by said three mold elements; wherein   said capacity of said intermediate cavity is larger than the capacity of said final cavity and foaming said charged molding material occurs in said final cavity, thereby forming a foam injection-molded part.   
     
     
       2. A foam injection molding method according to claim 1, wherein said two mold elements which are not moved during on intermediate cavity expanding and contracting step respectively have cavity forming surfaces, portions of said two cavity forming surfaces which lie in the vicinity of a line of intersection of said two cavity forming surfaces being not parallel to a direction of movement of said two mold elements. 
     
     
       3. A foam injection molding method according to claim 1, wherein said two mold elements which are not moved during said intermediate cavity expanding and contracting step respectively have cavity forming surfaces, portions of said two cavity forming surfaces which lie in the vicinity of a line of intersection of said two cavity forming surfaces being curved surfaces.